The synthesis pathway that is used exclusively in technical applications starts out from methylmercaptopropionaldehyde (MMP) which by addition of HCN is converted into the corresponding cyanohydrin (MMP-CH) which is then firstly catalyzed with sulfuric solution to form MHA amide and in a further step is hydrolyzed to form the hydroxy acid MHA, the sulfuric acid being converted into ammonium hydrogensulfate and, optionally, ammonium sulfate. ##STR1##
Starting from MHA hydrolysate, which consequently contains, in addition to corresponding portions of water, ammonium hydrogensulfate/sulfate, there are various methods for isolating MHA, which are described in summary form in German published application DE-OS 19 524 054 and in DE-PS 4 428 608.
These methods involve either solvent extraction or precipitation steps or a combination of both for the purpose of separating MHA from the salt that has been formed at the same time.
With each of these processes, in addition to the product MHA a portion of ammonium hydrogensulfate/sulfate corresponding to the quantity of sulfuric acid previously introduced is produced in the form of a solution or in a more or less pure solid grade. Since production of MHA is of the order of about 20,000 to 300,000 tons per annun per plant, quantities of sulfate salt also accumulate in quantities of approximately the same magnitude, which hitherto had to be disposed of in an expensive manner or dumped. However, in particular the dumping of such gigantic quantities of salt is unjustifiable from the ecological point of view and is also not economically sensible as a result of further increasing charges. With a view to utilizing the waste salts it is therefore particularly desirable to recover sulfuric acid from the sulfates and to return the sulfuric acid into the MHA process.
A quite similar composite method is already known from the technical process for preparing methyl methacrylate (MMA), as described, for example, in U.S. Pat. No. 3,549,320. In this case acetone cyanohydrin is first converted with, sulfuric acid into methacrylamide sulfate and subsequently with methanol into methyl methacrylate and ammonium bisulfate. After separation of the MMA, the residue containing sulfuric acid and ammonium bisulfate is transformed into sulfuric acid in a cracking contact plant. This method, which is known from the state of the art, is proposed in U.S. Pat. No. 5,498,790 with a view to the utilization of waste salt in the MHA process. In this connection the raffinate solution arising from the MHA extraction, principally containing ammonium bisulfate or ammonium sulfate, water and small quantities of organic by-products, is firstly subjected to combustion in a cracking furnace together with a fuel at about 900 to 1,200.degree. C. to form SO.sub.2, N.sub.2, O.sub.2, CO.sub.2 and H.sub.2 O: EQU 2NH.sub.4 HSO.sub.4 +1/2O.sub.2 .fwdarw.N.sub.2 +2SO.sub.2 +5H.sub.2 O
After cooling to 35.degree.-45.degree. C. with a view to condensation of portions of water and subsequent reheating together with oxygen, the hot combustion gas containing SO.sub.2 is conducted into a contact furnace and the SO.sub.2 is oxidized there on the V.sub.2 O.sub.5 contact substance at temperatures of at least 420.degree. C. to form SO.sub.3 : EQU SO.sub.2 +1/2O.sub.2 .fwdarw.SO.sub.3.
However, the economic implementation of this method is tied to minimum concentrations of SO.sub.2.
The contact gas containing sulfur trioxide is absorbed in oleum in accordance with a long-established procedure and, as a result of subsequent dilution with water, sulfuric acid is produced having a concentration of about 65 wt % that is needed for the MHA process.
The disadvantages of this method which is described in U.S. Pat. No. 5,498,790 consist in the elaborate and complicated process technology for the contact furnace, wherein the stream of cracking gas containing SO.sub.2 has to be conducted with oxygen over several contact trays and, after intermediate absorption of the SO.sub.3 which has already formed, conversion of the remaining sulfur dioxide is brought about on the last tray by intermediate cooling and reheating to the reaction temperature.
Further disadvantages are the high reaction temperature of over 420.degree. C., which in addition is attained by reheating the SO.sub.2 combustion gas which has cooled down to 35.degree.-45.degree. C., and also the non-quantitative conversion of the SO.sub.2 portion in the course of catalytic oxidation in the contact furnace, which results in a residual SO.sub.2 content in the waste gas leaving the contact furnace. This portion has to be lowered to the permitted legal limits by means of a suitable aftertreatment. The method described in U.S. Pat. No. 5,498,790 preferably operates with a minimum concentration of 70 wt % of sulfate salt in the input stream of the combustion furnace, since a lower concentration results in an additional demand for fuel for evaporation of the high proportion of water and in higher inert-gas portions consisting of CO.sub.2, N.sub.2 etc. In this case the SO.sub.2 concentration falls so considerably that the contact furnace can no longer be operated effectively. In particular, solutions containing ammonium as described in U.S. Pat. No. 4,912,257, the maximum sulfate concentration of which is clearly below 70 wt %, can be introduced in the form of a suspension only after further concentration. In operation this leads to greater difficulties such as blockages, for example, and is consequently not very practicable (cf. U.S. Pat. No. 5,498,790, bottom of col. 14 and top of col. 15).
Further disadvantages consist in the use of a heavy-metal catalyst that is tied to sufficiently high concentrations of SO.sub.2, and the risk resulting therefrom of contamination of the product with catalyst residues and also with NO.sub.x which can be formed as a result of catalytic oxidation at the stated reaction temperatures. In the course of returning the sulfuric acid into the MHA process these contaminants may find their way into the MHA end product which is employed as feedstuff additive. By their nature they are undesirable therein.